The petroleum industry has often sought new integration opportunities for its refinery products with other processes. One of the areas of interest concerns refinery gases that are currently used as a fuel. In addition, refineries are processing heavier crude oils and sulfur specifications for both diesel and gasoline products are becoming more stringent. Hydrogen can be used within hydrotreaters to remove sulfur, oxygen, and nitrogen and also within hydrocrackers to produce lighter and more parafinic oils. Consequently, refineries are looking for low cost sources of hydrogen.
While the refinery gases are a potential source of hydrogen, many refinery gas steams are not used either for their hydrogen content or to generate hydrogen through known reforming techniques due to a variety of economic and practical reasons. For instance, the economics for separating hydrogen from refinery gases that contain less than about 30% hydrogen are generally unfavorable. The direct use of refinery gases that contain significant amounts of olefins in steam methane reformer is not practical due to their high sulfur content, low pressure and variable olefin content. Moreover, the use of olefins as a feed to a reformer is problematical in and of itself due to the high probability of carbon formation that occurs with the use of higher order hydrocarbons at high temperatures.
While refinery gases also contain hydrogen, generally the hydrogen concentration within such refinery off-gases is too low for the hydrogen to be economically recovered using current available separation technologies.
The off-gas from a fluidic catalytic cracker unit (an “FCC”) is a prime source of olefins and is the focus of prior art efforts for olefin recovery. Generally, the feed stream has a light end content that includes hydrogen, methane, and nitrogen and a heavy end content that comprises ethane, ethylene, propane, propylene, and higher molecular weight hydrocarbons. The off-gas, as a feed stream, is compressed and then pretreated. During pretreatment, a number of operations are performed including, catalytic hydrogenation of acetylene to ethane, catalytic removal of residual molecular oxygen, caustic or amine scrubbing to remove carbon dioxide and hydrogen sulfide, and drying to reduce the water content to acceptable low levels in the parts per million range.
The treated feed stream is then fed to a light end separator to separate the methane, hydrogen and nitrogen from the higher order hydrocarbons including ethane, ethylene, propane and propylene. The light ends separated are used as a fuel gas which is sent to a fuel header. The fuel header is used to distribute fuel to steam generators to generate steam for driving distillation operations such as those used in acid gas removal and for electrical power generation. The olefins contained in the heavy ends are separated in stages, namely ethylene from ethane and propylene from propane.
When the light ends are separated from the heavy ends, the light ends contain hydrogen and nitrogen in nearly equal amounts. Therefore, nitrogen which has no heating value is being added to the fuel header. This is particularly disadvantageous in that the nitrogen absorbs heat from the combustion of the fuel. Furthermore, the piping and other equipment must be sized to accommodate the nitrogen.
There are a variety of known techniques that can be used to separate the nitrogen from the hydrogen. However, the hydrogen content is a relatively small portion of the overall light ends gas stream. This combination of low hydrogen content and high nitrogen content makes the recovery of hydrogen prohibitively expensive because the nitrogen invariably will have to be separated.
The light ends stream also contains methane which is of course a better feed stock than higher order hydrocarbons for reforming operations to generate hydrogen. However, the methane content of the refinery gases is sufficiently variable that it is often not practical to use the light ends stream for such purposes.
As will be discussed, the present invention provides a method of integrating the separation of olefins and the production of hydrogen from a refinery off-gas feed to economically and practically produce the hydrogen while allowing for the separation and disposal of nitrogen.